1. Field of the Invention
The present invention relates to a hearth structure of an oxygen-bottom-blowing converter for steel making, and more particularly a hearth structure of a converter superior in durability of bricks arranged around oxygen-blowing tuyeres of the converter.
2. Description of the Prior Art
There have generally been used burned or unburned bricks consisting mainly of magnesia, magnesia-dolomite or magnesia-chromite as bricks to be arranged around a tuyere. The unburned brick is produced by a process, wherein a mixture of starting materials consisting mainly of magnesia, magnesia-dolomite or magnesia-chromite is kneaded and shaped together with a binder, and the shaped article is hardened by heating the article at a relatively low temperature. The unburned brick is distinguished from burned bricks produced by sintering a starting material mixture at a high temperature.
Wearing of bricks arranged around tuyeres is mainly caused by the thermal spalling with the proceeding of blowing. This is ascertained by the fact that there are observed, in a brick after used, a large number of cracks parallel or vertical to the contact surface of the brick with molten steel and these cracked portions of the brick tend to be easily peeled off.
Bricks arranged around a tuyere are broken by thermal spalling more seriously than bricks arranged apart from the tuyere. The reason is probably as follows. The blowing oxygen is reacted with the components, particularly C, Si and Fe, in a molten steel at the tip of a tuyere to form a very high temperature portion (fire point) during the blowing, while the tuyere is cooled due to the simultaneous blowing of inert gas, thermally decomposable gas and the like. As the result, the molten steel convects and strikes bricks arranged around the tuyere and then flows back towards the fire point along the surface of the bricks. Further, the contact portion of bricks with a tuyere is cooled by the above cooling action. Therefore, a very steep temperature gradient is formed in the bricks arranged around the tuyere, which are therefore subjected to great thermal shocks.
It is clear that alleviation of such thermal shocks serves to decrease the wearing of brick by spalling and hence improve the durability of oxygen-blowing converter.
Carbon-containing bricks have hitherto been known as heat-resistant bricks.
For example, in some of electric furnaces, magnesia-carbon bricks are used in the hot spot, that is, in the peripheral portion of the furnace, which is faced to the arc-generating point and has a highest temperature. In this furnace, thermal conductivity of the bricks is improved by the enrichment of carbon content in the bricks and the radiation heat of arc is liberated to the outside of the furnace through the bricks by the effect of cooling subjected to the outer iron shell of the furnace, whereby the hot spot is eliminated, that is, the peripheral portion of the furnace exposed to the highest temperature is prevented from being excessively heated locally.
However, when a refining vessel having a tuyere for blowing oxygen, particularly an oxygen-bottom-blowing converter, is used, there is a risk that carbon in the brick is oxidized and consumed by a large amount of oxygen blown into a steel bath, and therefore the upper limit of 4.8% of carbon content in the unburned brick has been used in the practical operation. Such a carbon content is not sufficient to avoid the spalling of the bricks.
Bricks arranged around the tuyere of an oxygen-blowing converter are heated in a manner different from the heating by the radiation of arc in the electric furnace. That is, the bricks are heated by such a peculiar heating procedure that a molten steel heated up to a high temperature by a vigorous oxidation reaction for the molten steel attacks concentrically the bricks with a strong thermal shock under a steep temperature gradient due to the cooling action of inert gases and thermally decomposable gases introduced into the molten steel together with oxygen. The inventors of the present application have investigated the spalling breaks particularly seriously produced in the proximity of the tuyeres due to such thermal shocks to find that the bricks restrained between the tuyeres oppositely arranged in the hearth are subjected to greatly high thermal stresses due to the restraining effect of the tuyeres.
The inventors have made various investigations and experiments and found the following phenomena. In spite of that it has been considered the influence of oxygen blown into oxygen-blowing vessels, particularly into an oxygen-bottom-blowing converter, is considerably higher than the influence of oxygen in the case of an electric furnace, where only the peripheral portion of molten steel surface in the furnace is influenced by oxygen in the air, carbon in the bricks is hardly oxidized and consumed during the blowing of oxygen, because the oxygen blown into the molten steel is separated from the bricks by the molten steel.
Further, when a slag is removed from a converter after tapping of a molten steel, bricks arranged around a tuyere and in contact with the molten steel during the blowing are covered with the slag to prevent the direct contact of the brick surface with the air, and the oxygen blowing is carried out more advantageously than the heating in the electric furnace, where the hot spot in the furnace is always exposed to the oxidation atmosphere. In view of the above phenomena, the inventors have attempted to use magnesia-carbon or magnesia-dolomite-carbon bricks as bricks to be arranged around a tuyere, and found that not only the above described bricks are practically applicable contrary to the anticipation by the conventional technic, but also a furnace using the bricks can be used in a repeating number of about two times that in a furnace provided in the conventional bricks arranged around a tuyere.